This invention relates to clutch mechanisms for transmitting drive from a vehicle engine to a transmission gearbox, and more particularly to self-modulated clutches which automatically engage and disengage in response to engine speed changes.
Prior U.S. Pat. No. 3,508,450 discloses an advantageous vehicle transmission system having a change speed gearbox of the so-called "manual" type, but in which the operator need not manipulate a clutch pedal or the like in order to accomplish a shift. Movement of the operator's shift lever activates a pneumatic control system which then automatically disengages input and output clutches at opposite ends of the gearbox, brakes the gears to a stationary condition, performs the necessary changes of gear engagements, and then re-engages the input and output clutches to re-establish drive through the system.
In the absence of a clutch pedal or the like, the operator has no means for deliberately disconnecting the engine from the vehicle wheels while temporarily halted at road intersections or for other reasons. Similarly, the operator has no means for manually regulating or modulating the rate of clutch engagement at start-up of the vehicle or in order to reduce shifting shocks. Instead, the system performs both of these functions automatically as the input clutch is provided with a self-modulating mechanism. In particular, springs are present to retract the pressure plate of the input clutch from the clutch discs when engine speed drops to the idling range. This automatically disengages the input clutch when an operator removes his foot from the accelerator pedal of the vehicle or takes equivalent action through some other form of throttle or speed governor control. When the operator subsequently reaccelerates the engine, a rising fluid pressure is generated by centrifugal effects in a volume of lubricant within the clutch. The rising fluid pressure overcomes the disengagement springs and exerts a gradually increasing force on the pressure plate to re-engage the clutch in a modulated manner.
Prior U.S. Pat. Nos. 3,667,583, 3,667,309 and 3,744,608 disclose improved forms of the clutch described above, while prior U.S. Pat. No. 3,570,636 and copending application Ser. No. 452,116 of Phillip S. Webber et al, filed Mar. 18, 1974 now U.S. Pat. No. 28,710 and entitled CONTROL SYSTEM FOR SHIFTING A DRIVE TRANSMISSION, describe pneumatic controls for this type of transmission system including components which automatically force temporary disengagement of the input clutch in the course of a gearshift without regard to engine speed.
Transmission input clutches, including self-modulated clutches, may under certain conditions exhibit an undesirable drag effect while in the disengaged condition. The disengagement tends to be less than complete as some torque continues to be transmitted through the clutch when the pressure plate is retracted from the clutch discs due to frictional or viscous lubricant coupling of adjacent clutch discs. This effect is particularly troublesome in a vehicle equipped with a transmission system of the kind described above in which self-disengagement of the clutch at engine idling speed is relied upon to prevent powered movement of the vehicle when it is temporarily stopped with the engine running. Excessive drag at that time may cause the vehicle to creep and may force the operator to apply the vehicle brake in order to remain stationary. The problem tends to be most severe at start-up in cold weather as the lubricant is relatively viscous at such times.
In the case of manually operated or non-self-modulating clutches, it is known that drag may be reduced by stopping or reducing lubricant flow into the clutch during disengagement as exemplified by prior U.S. Pat. Nos. 3,773,157 or 3,823,802 for example. However, counteraction of drag in self-modulated input clutches of the kind discussed above is complicated as such clutches require a flow of oil not only to lubricate and cool clutch components but also to maintain the self-modulating mechanism charged with fluid. Heretofore, the oil flow into input clutches of this particular kind has either been constant at all stages of clutch operation or, in the case of U.S. Pat. No. 3,570,636 in particular, the oil flow into the clutch has been deliberately increased when the clutch is disengaged to provide increased cooling of the clutch discs during the subsequent engagement which normally occurs at the conclusion of a gearshift. Prior U.S. Pat. No. 3,570,636 includes a proportioning valve which normally distributes 80% of the available oil flow to the gearbox and 20% to the input clutch but temporarily reverses this distribution during the shift.
Counteraction of excessive drag in this kind of transmission system is further complicated by the fact that a controlled limited degree of drag may be deliberately provided for at least at certain specific stages of operation, as described at length in prior U.S. Pat. No. 3,744,608. Since the gears of the associated gearbox are shifted in a stationary or near stationary condition and no synchronizers are provided, it is possible that gear tooth abutments may occur as a shift is attempted. To remove such tooth abutments in order to assure gear engagement, springs may be present in the input clutch to apply a light pressure to the clutch discs prior to the time that full engagement pressure is applied at the conclusion of the gearshift. This gently turns the abutted gears, enabling completion of the desired engagement prior to the time that full torque is reapplied to the gearbox at the conclusion of the gearshift. This "roll-over" torque during a shift period should have a controlled, predetermined magnitude in order to be most effective. The occurrence of a variable uncontrolled additional amount of drag at this stage of operation can delay the desired gear engagements and cause spinning, raking and excessive gear wear.